The present invention relates to a filter for liquid and a method of filtering liquid using the filter. More specifically, the invention relates to a filter for liquid capable of removing dust of the order of submicrons contained in the liquid and a method of filtering the liquid.
It is required to remove even fine dust of smaller than 1 .mu.m from hydraulic fluids lubricating oils and electro discharge machining oils. However, mechanical filters such as paper filters, non-woven fabric filters and granular filters can remove dust of sizes of down to 5 .mu.m at the smallest but can no more remove dust of the order of submicrons. With the above-mentioned conventional mechanical filters, furthermore, if the mesh is decreased to increase the filtering efficiency, the pressure drop increases and the clogging develops at an early time causing the life of the filter to be shortened. Using the conventional mechanical filters, therefore, it is not possible to effect the precision filtration of the order of submicrons.
In order to carry out precision filtration, Japanese patent application Kokai publication No. 58-61845 proposes means according to which a pair of electrodes are provided in a solution to be filtered to assist the mechanical filter, and a voltage is applied across the electrodes to establish an electric field, so that dust in the liquid is collected or is migrated in the solution by the electric force of electric field. However, the method of providing the pair of electrodes as an auxiliary means necessitates additional facilities such as voltage generator and the like, inevitably involving disadvantage in handling the device and in manufacturing cost. Furthermore, even when it is attempted to carry out a higher degree of precision filtration, the object is never achieved as far as the method of providing the auxiliary electrodes is used in combination with the aforementioned conventional mechanical filters.
In recent years, there has been proposed the use of an air filter which is obtained in the form of electret fibrous sheet or an aggregate of pellets, the air filter being used for filtering dust suspended in the air. However, no example has been reported for using the above filter for filtering the liquid. The reason why the above filter is not used for filtering the liquid is attributed to that the electric force of electret does not effectively act in the liquid due to electric shielding effect of the liquid.